Reverse osmosis treatment process comprises applying a pressure higher than osmotic pressure to separate a solvent from solutes through a reverse osmosis membrane, thereby concentrating the solutes. In general, with an increase of an applied pressure in the range from 2 to 6 MPa, which is within the limit of pressure tightness of the membrane and apparatus, the throughput capacity per unit area of the membrane is increased and the degree of concentration of the solutes can also be increased.
Therefore, when using a multistage continuous reverse osmosis apparatus, which comprises a plurality of reverse osmosis modules connected in series and is operated so as to increase the concentration of solutes successively as a solution to be treated goes downstream, the permeate flux can be kept large when the apparatus is operated by applying to all modules their respective maximum allowable pressures. Accordingly, such an operating method has heretofore been considered an efficient method.
The present inventors have continued investigation on reverse osmosis treatment of dairy products. When a conventional multistage continuous reverse osmosis apparatus was operated for cheese whey by applying to all modules their repective maximum allowable pressures, the present inventors observed the lowering in throughput capacity because of a decrease in permeate flux of the module in each stage with the lapse of time. Such decrease of permeate flux is observed in treatment of other foods such as milk, skim milk, fruit juices, soybean milk and the ultrafiltrated permeates thereof. Skim and the ultrafiltrafiltrated permeates are reported in Hiddink et al., J. Diary Sci, 63, 204 (1980). Cheese whey has been tested by the present inventors, and reported in Lim, T. H., et al., J. Dairy Sci., 54, 306 (1971), and Smith, B. R., Aust. J. Dairy Tech., 33, 57 (1978).
In order to overcome such a decrease of the permeate flux, changing the pH of a solution to be treated and/or introducing a step of frequently washing a treating apparatus has been considered, but these approaches are not always sufficient. Further, in the case of a continuous reverse osmosis treatment process, when successive processes are constructed by carrying out thereafter a cooling procedure or another procedure, a change in the throughput capacity of a reverse osmosis apparatus forces the capacity of the subsequent process to be changed concomitantly, which is a serious problem. A decrease in permeate flux indicates adhesion of solutes onto membrane surfaces and fouling thereon. In processing foods, it is also a serious problem in that frequent washing of the apparatus is necessary.
An object of this invention is to provide a novel reverse osmosis treatment process which can solve these problems.